After several hovercraft accidents at Koala Base, Australia, and a Mysteron threat speaking of a traitor in Spectrum, Captains Scarlet and Blue are sent to the base to determine exactly who the traitor is. To avoid arousing any suspicions, they are giving lectures to the cadets stationed there.
The Senior cadet, who survived the last accident, is suspicious of Scarlet and makes an attempt on his life by setting a fire in the room the Captain shares with Blue. Scarlet awakes just in time to save his and Blue's lives.
The Senior cadet's partner, on the other hand is more suspicious of his field partner, whereas Scarlet and Blue are looking at the base commander as the main suspect.
Friction builds as the four are posted to the first hovercraft patrol of the area. Disaster strikes again and Scarlet orders everyone to abandon ship whilst he retrieves the black box, which would tell them what went wrong.
Eventually it is discovered that the real traitor was a component in the hovercrafts' construction, which malfunctioned during use, causing the explosions. A fatal error which had occurred four years previously, but had sent Spectrum into a paranoid frenzy.